The present invention relates to a non-asbestos, non-metallic fibrous base material comprising a first layer of fibrillated aramid fibers, (and optionally cotton fibers, carbon fibers, carbon particles and a filler material, such as diatomaceous earth) and a second layer of friction modifying particles. The invention further relates to a composite friction material comprising the above described fibrous base material impregnated with a phenolic resin or a phenolic based resin blend.
The friction material of the present invention has improved anti-shudder characteristics and improved strength, porosity, wear resistance and noise resistance.
New and advanced transmission systems and braking systems are being developed by the automotive industry. These new systems often involve high energy requirements. Therefore, the friction materials technology must be also developed to meet the increasing energy requirements of these advanced systems.
In particular, a new high energy type friction material is needed. The new high energy friction material must be able to withstand high speeds wherein surface speeds are up to about 65 m/seconds. Also, the friction material must be able to withstand high facing lining pressures up to about 1500 psi. It is also important that the friction material be useful under limited lubrication conditions.
The friction material must be durable and have high heat resistance in order to be useful in the advanced transmission and braking systems. Not only must the friction material remain stable at high temperatures, it must also be able to rapidly dissipate the high heat that is being generated during operating conditions.
The high speeds generated during engagement and disengagement of the new transmission and braking systems mean that a friction material must be able to maintain a relatively constant friction throughout the engagement. It is important that the frictional engagement be relatively constant over a wide range of speeds and temperatures in order to minimize xe2x80x9cshudderingxe2x80x9d of materials during braking or the transmission system during power shift from one gear to another. It is also important that the friction material have a desired torque curve shape so that during frictional engagement the friction material is noise or xe2x80x9csquawkxe2x80x9d free.
In particular, transmission and torque-on-demand systems incorporate slipping clutches mainly for the fuel efficiency and driving comfort. The role of the slip clutch within these systems varies from vehicle launching devices, such as wet start clutches, to that of a torque converter clutches. According to the operating conditions, the slip clutch can be differentiated into three principle classes: (1) Low Pressure and High Slip Speed Clutch, such as wet start clutch; (2) High Pressure and Low Slip Speed Clutch, such as Converter Clutch; and (3) Extreme Low Pressure and Low Slip Sped Clutch, such as neutral to idle clutch.
The principal performance concerns for all applications of the slip clutch are the prevention of shudder and the energy management of the friction interface. The occurrence of shudder can be attributed to many factors including the friction characteristics of the friction material, the mating surface""s hardness and roughness, oil film retention, lubricant chemistry and interactions, clutch operating conditions, driveline assembly and hardware alignment, and driveline contamination. The friction interface energy management is primarily concerned with controlling interface temperature and is affected by the pump capacity, oil flow path and control strategy. The friction material surface design also contributes to the efficiency of interface energy management.
Previously, asbestos fibers were included in the friction material for temperature stability. Due to health and environmental problems, asbestos is no longer being used. More recent friction materials have attempted to overcome the absence of the asbestos in the friction material by modifying impregnating paper or fiber materials with phenolic or phenolic-modified resins. These friction materials, however, do not rapidly dissipate the high heat generated, and do not have the necessary heat resistance and satisfactory high coefficient of friction performance now needed for use in the high speed systems currently being developed.
The present invention is an improvement over the Seitz U.S. Pat. No. 5,083,650 reference which involves a multi-step impregnating and curing process; i.e., a paper impregnated with a coating composition, carbon particles are placed on the paper, the coating composition in the paper is partially cured, a second coating composition is applied to the partially cured paper, and finally, both coating compositions are cured.
In other friction materials, metallic fibers combined with carbon materials were included in the friction material for wear resistance. For example, Fujimaki et al. U.S. Pat. No. 4,451,590 describes a friction material having metallic fibers, filler, carbon particles, carbon fibers and phenolic resin. However, the metallic based friction materials do not have sufficient porosity and compressibility to be capable of high fluid permeation capacity during use. Also, the metallic based friction materials are not sufficiently resilient or elastic, yet resistant to compression set to be capable of withstanding high facing lining pressures of up to about 1500 psi (approximately 105 kg/cm2). The metallic based friction material also is not capable of withstanding high surface speeds of up to about 65 m/second which are generated during engagement and disengagement of the new transmission and braking systems.
The present invention is also an improvement over earlier co-owned patents by the assignee herein, BorgWarner Inc., for friction materials. In particular, U.S. Pat. No. 5,998,307 relates to a friction material having a base impregnated with a curable resin where a porous primarily layer comprises at least one fibrous material and a secondary layer comprises carbon particles covering at least about 3 to about 90% of the surface of the primary layer. The U.S. Pat. No. 5,858,883 relates to a base material having a primary layer of less fibrillated aramid fibers, synthetic graphite, and a filler, and a secondary layer comprising carbon particles on the surface of the primary layer. U.S. Pat. No. 5,856,224 relates to a friction material comprising a base impregnated with a curable resin. The primary layer comprises less fibrillated aramid fibers, synthetic graphite and filler; the secondary layer comprises carbon particles and a retention aid. The U.S. Pat. No. 5,958,507 relates to a process for producing the friction material where at least one surface of the fibrous material which comprises less fibrillated aramid fibers is coated with carbon particles and a retention aid when at least 3 to 90% of the surface, impregnating with a phenolic or modified phenolic resin and curing. The U.S. Pat. No. 6,001,750 relates to a friction material comprising a fibrous base material impregnated with a curable resin. The porous primarily layer comprises less fibrillated aramid fibers, carbon particles, carbon fibers, filler material, phenolic novoloid fibers, and optionally, cotton fibers. The secondary layer comprises carbon particles which cover the surface at about 3 to about 90% of the surface.
It is to be understood that the various primary layers described in the above-referenced BorgWarner patents are useful with the present invention and are fully incorporated herein by reference.
In addition, various base materials, as described in commonly owned BorgWarner Inc. U.S. Pat. Nos. 5,753,356 and 5,707,905 (which describe base materials comprising less fibrillated aramid fibers, synthetic graphite and filler) are also contemplated as being useful with the present invention and as such are fully incorporated herein by reference. Yet another commonly owned patent, U.S. Pat. No. 6,130,176, relates to non-metallic fibrous base materials comprising less fibrillated aramid fibers, carbon fibers, carbon particles and filler and is also considered to be useful as a primary layer in the present invention and is fully incorporated herein by reference.
In order for friction materials to be useful in xe2x80x9cwetxe2x80x9d applications, the friction material must have a wide variety of acceptable characteristics. The friction material must be resilient or elastic yet resistant to compression set, abrasion and stress; have high heat resistance and be able to dissipate heat quickly; and, have long lasting, stable and consistent frictional performance. If any of these characteristics are not met, optimum performance of the friction material is not achieved.
It is also important that a suitable impregnating resin be used with the fibrous base material in order to form a high energy application friction material. The friction material must have good shear strength both when saturated with the wet resin during impregnation and when saturated with brake fluid or transmission oil during use.
It is also important, under certain applications, that the friction material have high porosity such that there is a high fluid permeation capacity during use. Thus, it is important that the friction material not only be porous, it must also be compressible. The fluids permeated into the friction material must be capable of being squeezed or released from the friction material quickly under the pressures applied during operation of the brake or transmission, yet the friction material must not collapse. It is also important that the friction material have high thermal conductivity to also help rapidly dissipate the heat generated during operation of the brake or transmission.
As far as is known, there is no disclosure of an anti-shudder friction material for use in transmission systems which includes a fibrous base material comprising an aramid-type fiber first layer and a secondary layer of friction modifying particles, including, in particular, silica materials, carbonized carbon particles and/or resin powders.
Accordingly, it is an object of the present invention to provide an improved friction material with reliable and improved properties compared to those of the prior art.
A further object of this invention is to provide friction materials with improved anti-shudder characteristics and high thermal conductivity, porosity strength, and noise resistance.
As a result of extensive research in view of the need for a better friction material, a friction material with improved characteristics has been developed by the invention. The present wet friction material is useful in xe2x80x9cwetxe2x80x9d applications where the friction material is xe2x80x9cwettedxe2x80x9d or impregnated with a liquid such as brake fluid or automatic transmission fluid during use. During use of the xe2x80x9cwetxe2x80x9d friction material, the fluid is ultimately squeezed from or is impregnating the friction material. Wet friction materials differ greatly, both in their compositions and physical characteristics from xe2x80x9cdryxe2x80x9d friction materials.
In order to achieve the requirements discussed above, many materials were evaluated for friction and heat resistant characteristics under conditions similar to those encountered during operation. Both commercially available brake linings and transmission materials were investigated and proved not to be suitable for use in high energy applications.
The present invention is especially useful in brakes and in clutch applications. The present invention provides a fibrous base material with an optimum amount of friction modifying particles deposited on the surface of the fibrous base material. Preferably, the friction modifying particles are deposited on the fibrous base material during the process for making the fibrous base material.
The present invention is especially useful with a primary layer comprising a fibrous base material comprising aramid fibers alone and/or combinations of less fibrillated aramid fibers, cotton fibers, carbon fibers, carbon particles and at least one filler material and, optionally, other ingredients. In certain examples, the less fibrillated aramid fibers create a porous fibrous base material. The carbon fibers provide the friction material with good heat conduction such that the friction material has desired heat resistance. The carbon particles also provide the friction material with good friction characteristics such as a good or smooth xe2x80x9cfeelxe2x80x9d in shift and essentially noise or xe2x80x9csquawkxe2x80x9d free operation of the brakes and clutches. The presence of a small relative amount of cotton fibers provides the friction material with desired xe2x80x9cbreak-inxe2x80x9d characteristics such that there is little or no shudder occurring during the initial use of the friction material.
A secondary, or top, layer of friction modifying particles is deposited on the primary layer to form the fibrous base material. Various friction modifying particles are useful as the secondary layer on the fibrous base material. In particular, silica particles such as diatomaceous earth, celite, celatom, and/or silicon dioxide are especially useful. The surface friction modifying particles are present at about 0.2 to about 20%, by weight, and preferably about 2-10%, by weight, and most preferably about 3-5%, by weight, of the fibrous base material.
The fibrous base material can be impregnated using different resin systems. In certain embodiments, it is useful to impregnate the fibrous based material with a phenolic resin or a modified phenolic-based resin. In certain embodiments, when a silicone resin is blended or mixed with a phenolic resin in compatible solvents and that silicone-phenolic resin blend is used to impregnate a fibrous base material of the present invention, an especially useful high energy friction material is formed.